Whisker growth on Sn thin film accelerated under gamma-ray induced electric field

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1 Whisker growth on Sn thin film accelerated under gamma-ray induced electric field Diana Shvydka, 1 Morgan Killefer, 2 Vamsi Borra, 3 Daniel G. Georgiev, 3 Victor G. Karpov, 2 E. Ishmael Parsai 1 1 Dept. of Radiation Oncology, 2 Dept. of Physics and Astronomy, 3 Dept. of Electrical Engineering and Computer Science, University of Toledo, Toledo OH, USA 1

2 Outline Introduction Preliminary studies Experimental details Samples Irradiation source 192 Ir specs Results: SEM images, whisker density, and whisker length Conclusions 2

3 Introduction We have previously investigated the effect of high-energy electron beam irradiation on whisker kinetics Evaluate the effect of g-ray irradiation on whisker kinetics One report of whisker growth on Sn-plated surface irradiated under 50keV x-rays, published ~50 years ago Possibility of developing non-destructive accelerated life-testing tool 3

4 Previous results: irradiation with 6MeV electron beam Samples irradiated under 6MeV electron beam of clinical linear accelerator Sn thin films (~300 nm) on glass Zn-plated steel floor samples (NASA computation center, courtesy of J. Brusse) placed on acrylic slab 4

5 Previous results: irradiation with 6MeV electron beam Irradiation time hours in sessions, to achieve dose 10-20kGy (SI unit of dose 1Gy=1J/kg) Observed enhancement in whisker growth and whisker lengths in irradiated samples compared to control samples Acceleration ratios ~200 found Main mechanism: substrate charging under electron beam Electrical measurements showed charging present only during irradiation 5

Previous results: irradiation with 6MeV electron beam (a) Sketch of the experimental setup for electrical characterization of sample charging: downward arrows represent the primary electron beam;

6 Previous results: irradiation with 6MeV electron beam (a) Sketch of the experimental setup for electrical characterization of sample charging: downward arrows represent the primary electron beam; upward arrows show the measured current of secondary electrons from the sample. The layers represent: 1-glass substrate, 2-conductive oxide, 3-tin, 4-spacer filled with plastic sheets, and 5-second (foil) electrode. (b) Current-voltage characteristics of the structure in (a) for different insulating spacers. The beam was repeatedly turned on and off, leading to the gaps (beam off) in the plots. 6

wire and moves along a catheter Dwells in pre-planned locations, a line source irradiation

7 Irradiation under Ir-192 source Clinical high-dose rate source Irradiation plan created Sample: Sn film on glass A small (~5mm) encapsulated radioactive source is attached to a wire and moves along a catheter Dwells in pre-planned locations, a line source irradiation geometry The dose (10 to 20kGy) was delivered in multiple irradiation sessions of 2-4 hours per 1kGy 7

Irradiation under Ir-192 source 192 Ir g-ray spectrum 192 Ir also emits b-particles, which are absorbed in source encapsulation Electric field Source average energy ~380keV Sn film thickness is too

8 Irradiation under Ir-192 source 192 Ir g-ray spectrum 192 Ir also emits b-particles, which are absorbed in source encapsulation Electric field Source average energy ~380keV Sn film thickness is too small to have significant number of interactions In 3mm thick glass ~7% of g-rays interact Compton scattering is predominant Some photoelectric effect processes Result: substrate charging and electric field in direction perpendicular to the film surface 8

9 Irradiation under Ir-192 source: are atomic displacements possible? The gamma rays average energy ~0.38 MeV, maximum energy <1 MeV The average Compton electron energy is ~40% of the maximum photon energy The energy transferred to an atom (mostly due to the Compton effect) is lower than the electron energy by ~4x10-6 (the ratio of the electron to atom masses). Consequently, the atom receives <4eV The energy is well below the displacement threshold energy estimated for Sn as 22.2 ev Other mechanisms of energy transfer are even less efficient No atomic displacements in Sn film under g-ray irradiation 9

10 Sn film samples Two Sn thin film samples studied Vacuum evaporated at RT RF-sputtered at RT Sn thickness ~ nm Deposited on 3mm-thick soda-lime glass covered with transparent conducting oxide (TCO, specifically, SnO 2 :F with nominal 15 Ohm/square sheet resistance; TEC-15 glass from Pilkington) 10

central area exposed to 100% dose and two side areas 25% dose Sample 2 connected strips Sn

11 Sample and irradiation geometry Scribes For a line source dose falls off with distance r as 1/r Sample 1 disconnected strips Continuous Sn film/tco scribed Scribe lines define central area exposed to 100% dose and two side areas 25% dose Sample 2 connected strips Sn film deposited in strips/tco continuous Sn strips exposed to 100%, 40%, and 20% dose levels 11

12 Length (um) Density, 1/mm2 Results: Sample 1 on glass/ scribed TCO % dose Scribe Source Position (mm) Scribe 100% dose 25% dose Whisker density after10kgy Whisker length after 10kGy 12

13 Results: Sample 1 on glass/scribed TCO Irradiated sample, 25% Irradiated sample, 100% Dose, kgy Whisker density, #/mm 2 Whisker length, µm Whisker density, #/mm 2 Whisker length, µm ± ± ± ± ± ± ± ± ± ± ± ±1.67 Sample had whiskers before irradiation (stored on the shelf for several months) Correlation between dose and whisker enhancement 13

14 Results: Sample 1 on glass/scribed TCO Data from the table are plotted Correlation between dose and whisker enhancement 14

15 Results: Sample 2 on glass/continuous TCO Irradiated sample imaged after deposition (before irradiation) after 10kGy after 20kGy after 20kGy+30 days on the shelf Control sample imaged after deposition, after 30 days shelf and 60 days shelf 15

16 Results: Sample 2 on glass/continuous TCO Irradiated sample Control sample Time Dose, kgy Whisker density, #/mm 2 Whisker length, µm Time, days Whisker density, #/mm 2 Whisker length, µm 30 hrs ± ± hrs ± ± ± ± hrs +30days ± ± ± ±0.12 Both whisker density and lengths were accelerated under irradiation No correlation between dose distribution and whisker enhancement 16

17 Results: Sample 2 on glass/continuous TCO Whisker density Whisker length Frequency counts irradiated vs. control Both whisker density (a) and whisker lengths (b) were significantly enhanced under irradiation 17

18 Results: Sample 2 on glass/continuous TCO Whisker density Whisker length Temporal evolution irradiated vs. control Both whisker density (a) and whisker lengths (b) were significantly enhanced under irradiation 18

19 Acceleration factor To quantify the effect of electric field on whisker growth we use whisker creation rate R whisker time density Distinguishing between E-field stimulated R STIM and spontaneous R SPON whisker growth rates, define acceleration ratio a R R SPON For our experiment irradiation time to 20kGy dose t R =60hrs, and shelf time for control t S =35days=840hrs, a=(378/60)/(101/840) 52 STIM 19

20 Sample Summary of observed correlations vs. sample type Radiation dose Local dose Average dose No Yes Yes Yes Average radiation dose correlates with whisker growth through induced electric field Field distribution depends on sample: connected cells/strips are equipotential 20

21 Conclusions Observed effect of accelerated Sn whisker growth under g-ray irradiation: both whisker densities and lengths are greatly enhanced Attributed to generation of electric charges in the insulating glass substrate supporting the Sn thin films.the charges create an electric field perpendicular to the film surface, providing conditions conducive to electrostatically driven whisker growth Field distribution depends on the sample: connected (e.g., through TCO underneath) cells/strips are equipotential The field acts mostly at the nucleation stage by diminishing the whisker nucleation barrier The observed acceleration factor of ~50; higher values are achievable Promising as a non-destructive readily implementable accelerated life testing tool for whisker propensity 21

22 References Morgan Killefer, Vamsi Borra, Ahmed Al-Bayati, Daniel G. Georgiev, Victor G. Karpov, I. Eshmael Parsai, Diana Shvydka, Whisker growth on Sn thin film accelerated under gamma-ray irradiation, arxiv: v2 [cond-mat.mtrl-sci] V. G. Karpov, Electrostatic theory of metal whiskers, Phys. Rev. Applied, 1, (2014) A. C. Vasko, G. R. Warrell, E. I. Parsai, V. G. Karpov, and Diana Shvydka, Electron beam induced growth of tin whiskers, J. Appl. Physics 118, (2015) D. Niraula, J. McCulloch, G. R. Warrell, R. Irving, V. G. Karpov, and Diana Shvydka, Electric field stimulated growth of Zn whiskers, AIP Advances, 6, (2016) W. C. Ellis, D. F. Gibbons and R. G. Treuting, Growth of metal whiskers from the solid, in Growth and Perfection of Crystals, Proceedings of an International Conference on Crystal Growth, Cooperstown, New York August 27-29, Edited by R.H.Doremus, B.W.Roberts, and D. Turnbull J.Borg, D.W.O. Rogers, Spectra and air-kerma strength for encapsulated 192 Ir sources, Med. Physics 26, 2441 (1999) F. H. Attix, Introduction to radiological physics and radiation dosimetry, Wiley

Electric field stimulated growth of Zn and Sn whiskers

Electric field stimulated growth of Zn and Sn whiskers J. McCulloch 1, D. Niraula 1, C. R. Grice 1, G. Warrell 2, A. Vasko 1, R. Irving 1, D. Georgiev 3, V. Borra 3, V. G. Karpov 1, E. I. Parsai 2, and